The primary goal of most borehole measuring systems is to determine the amount, location and producibility of hydrocarbon materials in a borehole. Hydrocarbons include oil, gas or both. There are many characteristics of the rock of the borehole wall which may be measured to determine hydrocarbon presence and producibility. These characteristics typically include rock texture and permeability, bulk density, electrical conductivity and bulk resistivity, just to name a few. These measurements are used to determine the amount and type of fluid in the rock. All of these measurable parameters are interdependent upon each other, so that it is desirable to measure as many of them as possible for a determination of hydrocarbon amount, type and producibility.
One example is neutron logging tools, which determine the bulk density and effective porosity of the rock. Another example is imaging devices, such as the formation microimager, which determines the microscopic variations in electrical conductivity. The microimager uses a contact method, where pads are physically pressed against the borehole wall for a direct electrical measurement of conductivity. Acoustic type devices are also known for mapping out the shape or diameter of the borehole wall, and for determining rock porosity.
Another important characteristic of the formation is its dielectric constant or electrical permittivity, which indicates the amount of electrostatic energy that can be stored in the material when a given voltage or electrical field is applied. The dielectric constant or permittivity of a material, referred to by the symbol .epsilon., is defined as .epsilon.=.epsilon..sub.x .epsilon..sub.0 where .epsilon..sub.0 is the permittivity of free space equal to 8.85.times.10.sup.-12 farads/meter (Inks units) and .epsilon..sub.r is referred to as relative permittivity. Examples of relative permittivities for particular materials is 1 for freespace and for gas, 2 for oil, between 4-10 for rock and about 80 for water. The dielectric constant for water is relatively independent of its salinity, whereas the conductivity of water varies considerably with salinity. Thus, the dielectric constant is a very important parameter for a determination of the hydrocarbons as distinguished from the quantity of water in the rock material. This is true, since the difference in conductivity of an oil-based mud versus a low salinity water-based mud is relatively small, whereas the difference of the dielectric constant is very pronounced.
Therefore, it is desirable to measure the dielectric constant of the rock and other materials of the formation in a borehole. This measurement can be used alone or may be combined with other measurements for a determination of the amount and producibility of hydrocarbon material in the rock of a borehole wall.